Encyclopedia of Health and Medicine

84 The Eyes
cause corneal ABRASIONS, particularly in dusty,
windy, or dry environmental conditions. Contact
lenses tend to accumulate protein deposits that
cause irritation. Most hydrophilic lenses are disposable,
so frequent replacement helps minimize
this as a problem. The optician may need to clean
or gently grind the surface of gas-permeable lenses
to clear away deposits. Contact lens hygiene,
including diligent HAND WASHING before handling
lenses and storing lenses in the appropriate disinfectant
solution, is essential.
Reading a Corrective Lens Prescription
Optometrists and ophthalmologists measure
refractive errors in diopters, a representational
scale of the distance in front of or behind the eye’s
lens the focal point of lightwaves entering the eye
must shift to allow the light waves to clearly focus
on the retina. The larger the diopter number, the
more the lens refracts, or bends, the light. A corrective
lens prescription represents the diopter as
minus or plus, according to the direction the correction
shifts the focal point. For example, the following
prescription corrects for myopia and
astigmatism:
OD – 5.75 + 0.50 164
OS – 6.00 +1.75 115
This prescription denotes different refractive
corrections for the right eye (OD) and left eye
(OS). The minus diopter is the spherical correction
for the myopia; the plus diopter is the cylindrical
correction for the astigmatism, and the last number
is the axis position for the cylindrical correction.
A lens with a strong correction may also
include an adjustment that tilts the lens to alter its
optical center, the prism, allowing a thinner lens
to deliver the same corrective power or to accommodate
a significant difference in the refractive
correction for each eye (anisometropia).
See also REFRACTION TEST; REFRACTIVE SURGERY;
VISION IMPAIRMENT.